Response of ocean plankton to ‘global warming’

Exploitation of a historical data archive

12-06-2013   In 2010, Canadian scientists suggested in Nature, that the amount of microalgal plankton in the ocean has decreased by 40 percent over the last century. This decrease was attributed to global warming.  Dutch oceanographers of the Royal Netherlands Institute for Sea Research NIOZ, the VU University and the Groningen University, have now presented results with a quite different outcome, in PLOS ONE of June 12th.

Since 1889, the colour at the surface of seas and oceans has been recorded by ship crews with a simple device consisting of 21 glass tubes filled with water with a colour scale from brown to clear blue. This instrument was designed by Forel and Ule more than a century ago. The authors show that the results obtained with this Forel-Ule scale are closely correlated with ocean colour measurements by satellite remote sensing, available since 1970; the global distribution of the Forel-Ule colour pattern  is shown to be related everywhere to that of the microalgal biomass at the surface.

Since 1889, this biomass has decreased in some regions, albeit less dramatically than the decline reported by the Canadian team.  Elsewhere, however, phytoplankton biomass has increased significantly, for example in the Atlantic Ocean. If global warming has been the cause of the changes it can safely be stated that the reaction of the ecosystem has not been uniform everywhere, as thought two years ago. The Canadians combined data from a number of different sources available in the literature and linked these together by model calculations, while the results now published in PLOS-ONE are based on a single observation series obtained with one and the same instrument. 

Trust in models, often used for extrapolations in climate research, has often been out of proportion in the past such that claims of prospective changes have had to be recanted. The question remains whether the observed trends (a greening and therefore plankton-richer Atlantic, and a bluing, progressively impoverished Pacific) will continue in the next decades – an important issue because the phytoplankton in seas and oceans is not only the basis of the food chain but also a major link in the cycling of carbon dioxide, a major greenhouse gas.

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Article:
Marcel R. Wernand, Hendrik J. van der Woerd, Winfried W. C. Gieskes, Trends in ocean colour and chlorophyll concentration from 1889 to 2000, worldwide. PLOS ONE.

Further information:
Dr M.R. Wernand, NIOZ, +31 222 369 417 or +31 222 314 394
Dr H. van der Woerd, VU-IVM, +31 20 598 95 65
Prof. dr W.W.C. Gieskes, Groningen University, +31 50 406 24 71
Nienke Bloksma, NIOZ Communicatie, +31 222 369 460 or +31 6 53 49 47 14

Picture above: A Forel-Ule observation of the North Sea  is taken from the NIOZ RV ‘Pelagia’.
Inset: The complete Forel-Ule scale (21 colour) covers natural water colours between indigo-blue to cola-brown.

Krümmel’s contoured North Atlantic sea colour map (1889). The sailing track of the steamship ‘National’ is shown in black. The colour (Wasserfarbe) was indicated as a percentage of a yellow potassium chromate solution added to a blue copper-sulphate solution.

Inset:
a: Representation of North Atlantic monthly and chlorophyll for the periods 1889-1999
b: The modelled chlorophyll for the period 1980-1991, compared to CZCS- (1979-1988) and SeaWiFS (1997-2000)

Over the investigated period we found that the Atlantic is greening (increasing of plankton) and the Pacific is bluing (declining of plankton).